Open and shut case: Is Ruth Rendell finally ready to open up about her puzzling personal life?

The bestselling author's latest thriller draws upon her own personal history.

The bestselling novelist Ruth Rendell walks across the large sitting-room of her house in Little Venice, west London, about to take her seat for the photo shoot, when she spots something on the mantelpiece that unsettles her.

Her eyes, piercing and blue, narrow ever so slightly as she fixes her gaze on the arrangement of objects either side of a pink glass vase. "Sorry, but this mantelpiece maddens me," she says. Her hands stretch out and, with the deftness of a magician, she quickly rearranges a small silver cat and bull around the vase so the effect is more pleasing. "I have an absolutely wonderful housekeeper, but she has no idea of this sort of thing," she says.

When I see her do this, I assume I will be led down the same uncomfortable, chilly path trodden by legions of journalists before me. Interviewers keen to uncover secrets about the two subjects she has vowed never to talk about – her unhappy childhood and the reason behind her decision to remarry her husband, Don, two years after their divorce in 1975 – have written about her obsession with control and her froideur at length.

The 83-year-old writer – famous for her Wexford series (made into a long-running ITV drama starring the late George Baker) and the rather more disturbing Barbara Vine novels – has often been portrayed as the Ice Queen of crime fiction. The Telegraph called her a "tough case to crack" – she has, according to its interviewer, a "dry and concise manner… She can seem a little cold and remote at first. It's because she does not expand on her answers and will float calmly on the silences that follow them." Another writer who interviewed her said, "It is only gradually you discern the fragile warmth that wanders through her like a jailed spirit."

Today, however, Baroness Rendell of Babergh (she has been a life peer in the House of Lords since 1997) is in a more loquacious mood. Despite the fact that she has had a bad night – she has a terrible cold, and the interview is punctuated by attacks of violent coughing – she is in a relaxed, unusually confessional frame of mind. Slim and trim, she has the figure of a woman half her age – a vegetarian, she maintains her nine-stone weight by working out on her cross-trainer and Pilates machines most mornings, and she tries to attend a Pilates class once or twice a week. Her hair is a flattering shade of ash-blonde and her inquisitive, cat-like face is framed by a pair of jet earrings given to her by her great friend, the novelist Jeanette Winterson: "Jeanette has given me a lot of earrings."

A few days before our appointment, Rendell had celebrated her birthday, and the basement sitting-room in which we talk is overflowing with flowers and cards, many featuring cats (she lives with a 13-year-old ginger tom called Archie). Outside, standing in the perfectly manicured back garden, is a tall, wooden, cat-proof bird table also given to her by Winterson. After delivering the bird table to Rendell's elegant, flat-fronted house overlooking the Regent's canal, Winterson took her out to supper at the Wolseley.

Rendell – named, alongside Winterson's adopted and biological mothers, as one of the dedicatees of the latter's bestselling memoir Why Be Happy When You Could Be Normal? – met the author of Oranges Are Not the Only Fruit in the mid-1980s when Winterson was 26. At that point, Rendell and her husband Don were living in a 450-year-old farmhouse in Suffolk that had a cottage in its grounds. "I used to lend it to writers to work in and Jeanette was one of the first people to use it," she says. "We shared an agent, Pat Kavanagh [who died in 2008] and Jeanette wrote [her fourth book] The Passion there." In her memoir, Winterson writes of Rendell, "She had been the Good Mother – never judging, quietly supporting, letting me talk, letting me be."

Rendell is one of those people you want to confide in immediately. In the early 1990s she told Anthony Clare, the presenter of Radio 4's In the Psychiatrist's Chair, "I am curious about people. I want to know their secrets… because I am the last person to whom I would tell a secret, people tell me their secrets." In a way, it could be said that her lifetime's work – she has written more than 70 books since her first From Doon with Death in 1964 – has been the stealthy accumulation and representation of other people's secrets.

Her novels, especially the ones written under the nom de plume of Barbara Vine – formed by her middle name and the maiden name of her great-grandmother – are full of clues to her methodology and her unique appeal. "Secrets, having them, creating them, keeping them and half-keeping them were the breath of life to her," she writes of a character in her first Barbara Vine novel, A Dark-Adapted Eye (1986). "Much of the interest and terror induced by great crimes is due, not to their abnormal content, but to that in them which is normal," she states in Asta's Book (1993).

The latest Barbara Vine novel is called The Child's Child and addresses two themes which seem to worm their way through a great deal of her writing: homosexuality and illegitimacy. The book has two storylines: one set in contemporary London which follows Grace, an academic working on the portrayal of illegitimacy in English fiction, her gay brother Andrew and his lover, James; the other begins in 1929 and centres on the unhappy lives of John, a homosexual, and his unmarried sister Maud, who falls pregnant.

"I felt that the treatment of these two strands [homosexuality and illegitimacy] was iniquitous," she says. "Both were very strongly present in society when I was young. Also, I had a cousin who was gay and who eventually died of an Aids-related illness in 1989. He was put through aversion therapy, which was pretty grim – this must have been in the 1970s, I think – and it was so horrible he ran away. Of course I knew he was gay – we were great friends as well as cousins. He was very unhappy and often very unpleasant – it sours the character, this sort of thing."

What does she, a Labour peer, think about the gay marriage bill, introduced by a Tory government? "I wonder what their motive is? I can't believe that David Cameron just wants it because he thinks it will be right – it doesn't sound a bit Conservative. I'm in favour of it, of course. I think they must have a hidden agenda, but I don't know what it could be. They didn't think it was right before – and lots of them [Conservatives] still don't think it's right."

In The Child's Child, one of the protagonists suggests that, while the British public might tolerate the gay community, the vast majority don't want to know what they do in the bedroom. If their attention is drawn to it then surely, he argues, they will be disgusted. Does she think this is true? "I think it's so significant," she says. "I think it's at the root of all prejudice against male homosexuality. That's why nobody really cares about lesbians, do they? It's penetration that bothers them."

As Rendell acknowledges, dead babies, miscarriages, and illegitimate mothers haunt her work. In A Dark-Adapted Eye – a compelling tale about the murderous relationship between two sisters, Vera and Eden – one of the minor characters, Anne, comments on the news that a maid has drowned herself after discovering that she was pregnant and a separate incident of the recent disappearance of a baby. "Have you done Macbeth at school?" she asks her friend, Faith, the narrator of the book. "Macbeth is full of babies and milk… It's really strange that a play like that which is full of horrors should have all that babies and milk stuff, isn't it?"

That final sentence is good summation of the disquieting power of the Barbara Vine novels, books described by Ian Rankin as "consistently better work than most Booker winners put together". Anthony Clare made a link between Rendell's preoccupation with "the obsessive, the psychotic, the psychopathic" and the family "as a place of secrets, untruths, distortions and defilements". Her childhood is off limits to interviewers, she has said – "No, I don't want to do that, not doing that, never describe my childhood, no, I'm not going to talk about that," she told one journalist – but today she has decided to offer a little.

"I was a child and in 1942 I was evacuated to the Cotswolds with my mother, who was a teacher – she went with her school," she says. "I lived in one house in the village and my mother was in the vicarage. The vicar had a maid-servant who was pregnant and she drowned herself in a pond. I don't think it was particularly uncommon. What would she have done with the baby? What would have happened to it? How could you cope with the disgrace? I can quite imagine it would have been too much for her – I'd probably have done the same. But I wonder that they told me – I was only 12. Perhaps it was my mother, I can't remember. I knew anyway and I knew why. I've never forgotten it and I put it in that book."

Ruth Rendell was born Ruth Barbara Grasemann on 17 February 1930 in South Woodford, Essex. Her parents – Ebba, her mother, who was Swedish-Danish, and Arthur, her English father – did not have a happy marriage; in fact it was, as Ruth says, "a great disaster".

Although both of them were teachers it seemed that they were ill-suited and incompatible: her mother, who came to Britain when she was 14 not speaking any English, was not the domestic type and apparently did not live up to her father's exacting standards. "I think they married because they were totally innocent, especially my father," she says. "They were obviously very unsuited to one another, and they married in the days when you didn't marry foreigners. My grandmother refused to go to the wedding. My mother's family experienced the same sort of difficulties as the people who came here from the Caribbean 50 years later. I think my father must have been very much in love, but he realised it wasn't going to work.

"Then my mother started to suffer from multiple sclerosis, but nobody knew what MS was then. My father didn't – and later he suffered a great deal of guilt over that. It was an awful business and very fraught. I felt exasperated with them because I felt people should – in the days when people didn't really get divorced – at least put up a show of getting on in front of their child."

Rendell grew away from her mother and tended to idolise her father (he, in addition to Rendell herself, is the model for the intelligent, sensitive Inspector Wexford). "I was not encouraged to grow towards her," she says. "That was very wrong. I think there were a whole lot of wrong things going on there." Would she say there was a link between the toxic emotions that originated in her family and the leakage and expression of them later in her books? "It looks like it, doesn't it?" she says. "But I don't feel it consciously. I think these things are very complicated anyway." Did the experience affect her own view of marriage? "I expect so, it must have done."

Ruth met her future husband, Don Rendell, when she was working as a journalist on the Chigwell Times and he was on the Stratford Express. Rendell was a parliamentary reporter and one of the fastest shorthand writers in the country. "Yet his own handwriting was just awful; nobody could read it," she laughs. She was 20 when they married and the couple lived in a small tithe cottage, "a nasty little house", in the back streets of Leyton, courtesy of Don's newspaper. When he left that job – he later worked for the Daily Mail – the couple were turfed out and had to live back with Ruth's parents. "We were not welcome at all," she says. "We lived there for months rather than years."

Ruth and Don managed to scrape enough money together to buy a place of their own. "But that was dreadful – we had three rooms and a sort of kitchen, but no bathroom; we had an outside lavatory. You don't mind that so much when you are very young, but it was pretty grim. Also, we had a tenant on the upper floor and they had a water accident, with all this water that poured through the ceiling. When I had my baby [Simon, born in 1953], we were living in one room down there. But I am not sorry I had that experience because I know what it's like to have lived like that." Today, she gives a substantial amount of her earnings away – she is vice-president of housing charity Shelter, and she also supports Kids for Kids, helping children in Darfur, and Little Hearts Matter, which raises money for children with only half a heart. "I often think what it was like not to have much money," she says. "I don't think it's good for people to be born into money and not know what it is never to have it."

In 1975 the couple divorced, but reunited and remarried two years later. This remains one of the central mysteries of Rendell's life. She told Anthony Clare that although she knew the reasoning behind the decision the subject was decidedly off limits. How does she deal with the question today? "I don't really talk about it," she says. "No, I think I'd rather not talk about it." Why is it off limits? "It's private," she says. "I don't really want to talk about it and then get more and more questions asked. No, I think I've said enough about that really." She told Clare that one day she would perhaps write her autobiography so as to prevent the revelation arising in an unauthorised biography. "Well, there isn't going to be one," she tells me of the planned autobiography. "They can [write a biography] after I'm dead, I don't really care, but I don't think they will. I suppose they could ask my son, but he wouldn't tell them anything he didn't want them to know."

Her husband died in 1999 from prostate cancer. "It was a great blow, but not a shock because he had been ill for a long time," she says. "I'm afraid he didn't go to a doctor in time. If he had done, he might even be alive today. But he wouldn't do it – in spite of my urging him."

Her son, Simon, who will be 60 this year, is a psychiatric social worker and lives in Colorado; he has two boys, Philip, 21, and Graham, 19, both of whom are at university in America. Does she wish she had had more children? "No, I don't think so," she says. "It's an awful admission; one is expected to want a lot of children." Was she a good mother? "You would have to ask him. I don't know, but I hope so. Not bad," she says, smiling.

There are no signs of the indomitable Baroness Rendell slowing down: she still gets up at six each morning and, after exercising and a light breakfast, she writes between 8.30am and 11am or 12pm. Afternoons are taken up by her work at the House of Lords. She has just finished another Wexford and now she is in a period of reflection about what to write next. "It won't be another Barbara Vine, if there will ever be another one," she says.

Ideally, she would like to die while writing. "Of course, one doesn't get the chance – people don't have deathbeds now, do they?" she says. "They get either drugged or given palliative care as they gradually disintegrate." She has pledged her support to a private member's Bill to legalise assisted suicide in this country. So would she, if the situation arose, opt for Dignitas? "I suppose if I say I support [the Bill], I would do that," she says. "The way I'm going on, it won't be long, will it?" she jokes as she coughs. "But I am very strong, very well and all my aunts lived into their nineties. You can't tell what may happen to you, but I don't see myself going to Dignitas. I just don't see it happening to me."

'The Child's Child' by Barbara Vine is published by Viking, priced £18.99